1. Field of the Invention (Technical Field)
This invention relates generally to the field of sampling target analytes in which the target analytes, themselves, are in vapor phase. More specifically, the invention relates to using solid-phase microextraction in sampling of target analytes present in low concentration in a carrier gas, especially where the target analytes, such as emanations from explosive materials, are present in a matrix, such as soil, which comprises gas, liquid and solid components.
2. Background Art
The world faces the growing problem of locating and remediating hidden explosives hazards such as buried land mines and abandoned unexploded ordnance in soil. An approach to the problem that has met with some noted success, and that holds the promise of future improvement, involves chemical sampling of gaseous emanations from explosive materials such as TNT in soil. These emanations may include molecules of the actual hidden explosive substances that penetrate and partition into soil gases or they may include other identifier or marker molecules that can be linked to the presence of explosives in the soil. Examples of such identifier or marker molecules may include chemical breakdown products or manufacturing impurities of the explosive materials of concern.
Challenges exist relating to detection and measurement of target analytes in this context. These challenges stem primarily from the extremely low concentration of explosive molecules or other marker substances typically present in soil gas. Sometimes such concentrations are below the level of parts per billion. Low concentrations can result in problems of long sampling times necessary to collect enough target analyte for accurate detection and/or quantitation. Therefore, strategies are needed for optimizing the collection of the explosive components or other target analytes so that sensitivity of detection is maximized and sampling time is minimized.
For purposes of this disclosure, detection of explosive substances is but one embodiment wherein the principles of the invention can be successfully applied. The methods and apparatuses described and claimed herein can be adapted and applied beneficially to a broad range of chemical sampling challenges wherein low concentration of target analyte makes accurate detection and quantitation difficult.
Solid-phase microextraction (SPME) techniques have been the subject of considerable study in recent years, and SPME is emerging as a favored method for sampling of low concentration explosives and other analytes. References describing SPME techniques, specifically as regards to explosives detection include xe2x80x9cTrace Analysis of Explosives in Seawater Using Solid-Phase Microextraction and Gas Chromatography/Ion Trap Mass Spectrometryxe2x80x9d, S. A. Barshick and W. H. Griest, Anal. Chem. 1998, 70, 3015-3020; xe2x80x9cTrace Explosives Signatures from World War II Unexploded Undersea Ordnancexe2x80x9d, M. R. Darrach, A. Chujian, and G. A. Plett, Environ. Sci, Technol. 1998, 32, 1354-1358; xe2x80x9cApplication of Solid-Phase Microextraction to the Recovery of Organic Explosivesxe2x80x9d, K. P. Kirkbride, G. Klass and P. E. Pigou, J. Forensic Sci., 1998, 43(1), 76-81. Each of the references cited above describes generally the use of SPME fibers. Typically, such fibers are fine (xcx9c0.25 mm OD) silica fibers coated with a thin layer of a sorbing material. SPME fibers are often coated with a sorbent chosen or engineered to have a high propensity to sorb certain analytes of interest. The fibers are exposed to a gaseous or liquid environment from which a target analyte sample is to be extracted. In general, low (near ambient) temperatures are required for optimal sorption of explosive gases from air.
After a sample is collected, the fiber can then be conveniently inserted into a gas chromatograph (GC) by placing the fiber into the inlet of a GC apparatus. One common way to accomplish this is to use a needle to puncture a septum covering the GC inlet, and a syringe plunger to push the fiber (containing sorbed analytes) through the needle into the GC apparatus. Next, the fiber is rapidly heated to drive off the analytes sorbed to the sorbent substance coating the fiber. The analytes are then swept into the GC column for normal separation and quantitation.
Typically, SPME sampling involves placing the SPME fiber in the headspace above a contaminated or potentially contaminated test subject material (for example, soil). Analytes then passively diffuse through the headspace and some ultimately adhere to the fiber. For gaseous samples of low concentration (such as in the case with explosives in soil gases), diffusion of the analytes through the gas to the SPME fiber can be a rate limiting step, resulting in long sampling times. This is especially true for instances wherein it is necessary for equilibrium to be reached, as is the case, frequently, in quantitation studies. xe2x80x9cSolid-Phase Microextractionxe2x80x9d, Z. Zhang, M. J. Yang and J. Pawliszyn, Anal. Chem. 1994, 66(17), 844A-853A; xe2x80x9cHeadspace Solid-Phase Microextractionxe2x80x9d, Z. Zhang and J. Pawliszyn, Anal. Chem. 1993, 65, 1843-1853.
SPME has been shown to successfully collect target analytes in low concentration in gases and liquids. An opportunity, however, exists for optimization of SPME techniques, and further, a need remains for an optimized method and apparatus for extracting target analyte substances from volumes of gases containing those substances in low concentration. The need is especially apparent as regards to overcoming problems associated with slow equilibration and long sample times.
It is noted that the assignee of this application, at the time the present application is made, also has a separate patent application (Ser. No. 09/205,158, Chambers, et al.) pending before the USPTO pertaining to a different use of chemical sorption in the context of detecting buried munitions. It is submitted, however, that the technology described and claimed in that application is distinct from the novel SPME techniques and apparatuses of the present disclosure, both in terms of theoretical principles and application.
The present invention provides a method and apparatus for sampling target analytes in which the target analytes, themselves, are in vapor phase or are suspended in a gas, as in the case of emanations from explosive materials present in soil. In one aspect, the invention uses a novel technique of solid-phase microextraction wherein traditional SPME fibers are omitted in favor of using a new SPME capillary. This technique is augmented in one described embodiment by using heating means (for example, microwave heating) to increase gas partitioning where analyte may be present either in gas and liquid, gas and solid, or gas, liquid and solid components present within the matrix to be analyzed. In another aspect, the invention utilizes the heating (such as microwave heating) to increase gas partitioning to enhance sample collection even where traditional SPME fibers are used.
Advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.